1. Field of the Invention
The present invention generally relates to a development device, a process cartridge, and an image forming apparatus such as a copier, a printer, a facsimile machine, or a multifunction machine capable of at least two of these functions that includes the development device.
2. Discussion of the Background Art
In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction devices including at least two of those functions, etc., include a latent image carrier on which an electrostatic latent image is formed and a development device to develop the latent image with developer.
There are two types of developer used in electrophotographic images forming apparatuses, namely, one-component developer consisting of magnetic or non-magnetic toner and two-component developer including toner and carrier particles. Recently, two-component developer has come to be widely used because its durability and image quality are better than those of one-component developer. Development devices using two-component developer (hereinafter “two-component development devices”) typically include a rotary cylindrical developer carrier (e.g., development sleeve) inside which a stationary magnetic field generator having multiple magnetic poles is provided to carry the developer on the development sleeve.
In certain known development devices, the magnetic field generator has five magnetic poles that can generate magnetic fields of sufficient strength for the development sleeve to carry the developer. The five magnetic poles include an attraction pole, a pre-development transport pole, a development pole, a release pole, and a post-development transport pole. The developer is attracted to a circumferential surface of the development sleeve at a position corresponding to the attraction pole (hereinafter “attraction portion”), and the pre-development pole generates a magnetic field for the development sleeve to transport the developer carried on the development sleeve to a development area or range facing the latent image carrier. The development pole contributes to latent image development in the development range, and the release pole contributes to separating the developer that has passed through the development range from the development sleeve. In this known configuration, the post-development transport pole is disposed between the development pole and the release pole and generates a magnetic field for the development sleeve to reliably transport the developer that has passed through the development range to the release position. In addition, in this known configuration, a developer regulator (e.g., doctor blade) is disposed facing the development sleeve between the attraction pole and the pre-development pole to adjust the amount of the developer carried to the development range.
With this configuration, processes of attracting the developer to the circumferential surface of the development sleeve, transporting the developer to the development range, developing the latent image with the developer, and releasing the developer from the development sleeve can be performed reliably. Alternatively, in certain known development devices, the magnetic field generator further includes a developer regulation pole disposed between the attraction pole and the pre-development pole, facing the developer regulator, and does not include the post-development transport pole.
At present, it is preferred that the development devices be more compact due to an increasing demand for more compact image forming apparatuses. The development devices can be more compact by using a development sleeve of reduced diameter.
However, in the known development devices, there in a practical limit to how much the diameter of the development sleeve can be reduced because it becomes difficult to reliably attract the developer to the development sleeve, transport the developer to the development range, develop the latent image with the developer, and release the developer from the development sleeve. Although magnets capable of generating a magnetic field of sufficient intensity are required to perform these processes reliably, the size of magnets increases as the intensity increases and therefore, it is difficult to reduce the diameter of the development sleeve inside which such large magnets are provided.
In view of the foregoing, there is a need to reduce the diameter of the developer carrier to make the development devices more compact while performing the above-described processes reliably, which the known image forming apparatuses fail to do.